The term cultural relativism gets used for more than one concept.

The term can refer to the practice of attempting to adopt the perspective of another culture in viewing that other culture, in viewing one's own culture, or in viewing some third culture. This practice is a healthy one. One should not simply presume that any deviation from one's own customs is deficient, nor even that one's own culture does everything at least equally as well as does any other culture. And, even in cases in which one's own culture is doing something better than does another, it is good to develop an understanding of why things seem as they do to people not from one's own culture.

But cultural relativism is also used to refer to the doctrine that every culture is every bit as reasonable as is every other culture. The rejection of presumption of superiority of one's own culture is replaced by a presumption of equality of cultures. This doctrine is madness, as is swiftly shown by exposing the self-contradiction in claiming that cultures that reject that very doctrine are every bit as reasonable as those cultures that embrace it.

Indeed, a cultural relativist of that sort could not even find logical standing to disagree with a single person who rejected cultural relativism. A culture is a set of customs, broadly conceptualized, of some group of persons who, if more than one person, are connected by interaction. A subculture is itself a culture, of a group contained within some large cultural group. When we say group, we naturally think of a set with more than one member, and some people might insist that the term culture cannot properly refer to customs peculiar to just one person; that insistence would be just a matter of arbitrary taxonomy; the customs of a group of two are no more magically different from the customs of single person than the customs of three are magically different from the customs of two.

Often times, people who advance that second notion of cultural relativism employ something of a motte-and-bailey argument. The motte is the earlier point, that one should not simply presume that any deviation from one's own customs is deficient, or even that one's own culture does everything at least equally well as any other culture. The bailey is that every other culture is just as good, overall or perhaps even in each particular. Of course, most of these cultural relativists will look-away from those cultures that make the relativists or their audiences uncomfortable. Mind you that a significant share of these relativists are not consciously employing a motte-and-bailey argument, and many of them are not conscious that they are averting their gazes from self-contradictions and from the cases of cultures that almost no one wants to defend. Most of these relativists just got thoughtlessly swept up and psychologically over-committed.

When the bailey that every other culture is just as good is unchallenged, it often is treated as a motte for a new bailey that is still worse. The idea that every other culture is just as good is quietly replaced with the idea that every other culture is at least as good, and then it is argued that some cultures are in some ways better than our own. Indeed some cultures are in some ways better than our own, if not necessarily in just any way that a particular critic might claim. But, coupling that point with the spurious proposition that these other cultures are in every other way at least as good, these relativists arrive at a conclusion that ours is necessarily an inferior culture — no better in any way, and worse in some.

Thus, the practice of seeking to free oneself from cultural presumption is perverted first into a new and foolish presumption, and thence into a sophistic attack on our culture.

I often hear or read someone objecting to judging an historical person or act by present moral standards. Although there seems to be some element of reasonableness entangled in this objection, it's very problematic.

It is especially problematic as expressed. Technically, we cannot judge anyone or anything at all, except by whatever may be our present standards. If we judge historical people and acts differently from how we do present-day people and acts, it is exactly because our present standards incorporate a recognition of historical context.

I don't see that the real issue is historical context as such, but context more generally. If we are to make allowances for historical person or acts, it is because of what informed them and what did not inform them; and, similarly, acts by persons in some present-day contexts are very differently informed from acts by other persons in other present-day cultures. As L.P. Hartley usefully noted, The past is a foreign country: they do things differently there.

Cultural relativism, in application to other places or to other times, is sensible when it warns one against presumption that one's own culture is doing things the only right way. (One's culture may be doing things a wrong way, or there may be other ways that are just as good.) But a cultural relativism that instead claims that something is automatically acceptable simply because it prevails in the culture of that place or prevailed in the culture of that time dissolves into nihilism because each person at each time and at each place is him- or herself a subculture.

And I think that some allowances should be made; but I think that too much allowance is often made.

For example, is the case against slavery now available really all that much better than the case that was available in America a few hundred years ago? Inverting that question, was the case against slavery available a few hundred years ago really much worse than the case available now? There is a sound argument, even to-day, for not waging war against slavery in the territories ruled by other states; and there may be a case for making treaties or even forming alliances with such states; but those are different practices from engaging in slavery or actively enabling slavery. Is there really a meaningfully better defense of the slavery of two hundred years ago than there would be of slavery now?

I   don't   think   so.

Nor do I think so for a great deal else that I am told not to judge by modern standards.